This invention relates to dental alloys and, in particular, to gallium and silver free, palladium-based, dental alloys for use in preparing essentially bubble-free, porcelain-fused-to-metal restorations.
Porcelain-fused-to-metal restorations consist of a metallic sub-structure coated with a veneer of porcelain. Over the years various alloys have been proposed for the sub-structure of these restorations. Many of the early alloys used gold with some platinum or palladium as the main alloy ingredients. However, with the increases and fluctuations in the price of gold and platinum in recent years, other alloys have come to play major roles in this area. One series of alloys which has gained general acceptance is based on nickel, chromium and beryllium as the main ingredients. Another series of alloys, with which this invention is concerned, is based on palladium as the dominant element.
Alloys suitable for use in porcelain-fused-to-metal restorations must satisfy a plurality of demanding conditions imposed both by the marketplace and by the physical and chemical requirements applicable to alloys for use in dental restorations. With regard to the marketplace demands, the alloy should have as low a price as possible. Specifically, it is important to avoid, if possible, the inclusion of expensive elements in the alloy. For example, large amounts of gold, such as the amounts used in U.S. Pat. No. 4,123,262 to Cascone or U.S. Pat. No. 4,205,982 to German, should not be included in the alloy because of both the high price of this element and the essentially daily fluctuations in its price. Similarly, high amounts of gallium, such as the amounts used in U.S. Pat. No. 4,387,072 to Schaffer, are preferably avoided because of the relatively high cost of gallium. Also, the use of gallium in dental alloys has in some cases been questioned for health reasons. Moreover, it is desirable to keep the amount of palladium in these alloys as low as possible in view of the relatively high cost of this element.
With regard to physical and chemical characteristics, the alloy should have a coefficient of thermal expansion such that the porcelain is under compression in the finished restoration. Further, during the porcelain firing process, the alloy must form a suitable protective oxide. The oxide should not cause discoloration of the porcelain and should be of the type which gives the alloy melting characteristics similar to that of pure gold. In this regard, it is important to avoid the inclusion of silver in the alloy because silver has a strong tendency to discolor a number of commercially available porcelains. Also, the alloy should have a high melting temperature so that castings made from the alloy will retain their shape during the porcelain firing process.
Of primary importance is the grain structure of the alloy. If the alloy has a good grain structure, it will have high elongation, tensile strength and toughness. These properties are important in avoiding "hot tearing" and in providing a casting with good burnishability.
Of similar critical importance is the alloy's resistance to the absorption of gas during manufacture, torch melting, casting and the porcelain firing process. If the alloy does absorb gases, these gases can be released during the porcelain application process to form bubbles in the porcelain. The presence of such bubbles makes the restoration unsuitable for implantation in a patient's mouth and thus requires a remaking of the restoration.